Induction cooking device

Abstract

An induction cooking device including at least one inductor positioned under a thermally strengthened glass plate, the composition of which glass is not of lithium aluminosilicate type, characterized in that the glass has the following characteristics: its thickness is at most 4.5 mm, the c/a ratio of the glass before strengthening is at most 3.0 after Vickers indentation under a load of 1 kg, c being the length of the radial cracks and a being the half-diagonal of the Vickers impression, the σ/(e·E·α) ratio is at least 20 K·mm −1 , or even 30 K·mm −1 , σ being the maximum stress generated at the core of the glass by the thermal strengthening in Pa, e being the thickness of the glass in mm, E being the Young's modulus in Pa and α being the linear thermal expansion coefficient of the glass in K −1 is provided.

The invention claimed is: 
     
       1. An induction cooking device comprising at least one inductor positioned under a thermally strengthened glass plate, wherein the glass plate is (a) an alumino-borosilicate glass or (b) a borosilicate glass, wherein:
 a thickness of the glass plate is at most 4.5 mm, 
 a c/a ratio of the glass plate before strengthening is at most 3.0 after Vickers indentation under a load of 1 kg, c being a length of radial cracks and a being a half-diagonal of the Vickers impression, and 
 a σ/(e·E·α) ratio is at least 20 K·mm −1 , σ being a maximum stress generated at a core of the glass plate by the thermal strengthening in Pa, e being the thickness of the glass plate in mm, E being a Young's modulus in Pa and α being a linear thermal expansion coefficient of the glass plate in K −1 . 
 
     
     
       2. The device of  claim 1 , wherein the thickness of the glass plate is at most 4 mm. 
     
     
       3. The device of  claim 1 , wherein the glass plate has a lateral dimension of at least 0.5 m. 
     
     
       4. The device of  claim 1 , wherein the product E·α of the Young's modulus and of the linear thermal expansion coefficient of the glass plate is between 0.1 and 0.8 MPa·K −1 . 
     
     
       5. The device of  claim 1 , wherein a lower annealing temperature of the glass plate is at least 500° C. 
     
     
       6. The device of  claim 1 , wherein the linear thermal expansion coefficient of the glass plate is at most 50×10 −7  K −1 . 
     
     
       7. The device of  claim 1 , wherein the c/a ratio of the glass plate before strengthening is at most 2.8. 
     
     
       8. The device of  claim 1 , wherein the maximum stress generated at the core of the glass plate by the thermal strengthening is at least 20 MPa. 
     
     
       9. The device of  claim 1 , wherein the the glass is a borosilicate glass. 
     
     
       10. The device of  claim 9 , wherein the glass comprises silica SiO 2  in a weight content ranging from 70% to 85%, boron oxide B 2 O 3  in a weight content ranging from 8% to 20% and at least one alkali metal oxide, the total weight content of alkali metal oxides ranging from 1% to 10%. 
     
     
       11. The device of  claim 10 , wherein the glass comprises, by weight:
 70 to 85% of SiO 2 , 
 8 to 16% of B 2 O 3 , 
 0 to 5% of Al 2 O 3 , 
 0 to 2% of K 2 O, and 
 1 to 8% of Na 2 O. 
 
     
     
       12. The device of  claim 1 , wherein the composition of the glass is an alumino-borosilicate glass. 
     
     
       13. The device of  claim 12 , wherein the glass comprises, by weight:
 45 to 68% of SiO 2 , 
 8 to 20% of Al 2 O 3 , 
 4 to 18% of B 2 O 3 , 
 5 to 30% of RO, and 
 at most 10% of R 2 O, 
 
       wherein RO is at least one alkaline-earth metal oxide selected from the group consisting of MgO, CaO, SrO and BaO, and R 2 O is an alkali metal oxide. 
     
     
       14. The device of  claim 1 , wherein a portion of the surface of the glass plate is equipped with an opaque or substantially opaque coating, or wherein an opaque material is positioned between the glass plate and internal elements of the device. 
     
     
       15. The device of  claim 1 , wherein the σ/(e·E·α) ratio is at least 30 K·mm −1 . 
     
     
       16. The device of  claim 1 , wherein the product E·α of the Young's modulus and of the linear thermal expansion coefficient of the glass plate is between 0.2 and 0.5 MPa·K −1 . 
     
     
       17. The device of  claim 1 , wherein a lower annealing temperature of the glass plate is at least 600° C. 
     
     
       18. The device of  claim 1 , wherein the linear thermal expansion coefficient of the glass plate is between 30 and 45×10 −7  K −1 . 
     
     
       19. The device of  claim 1 , wherein the c/a ratio of the glass plate before strengthening is at most 2.5. 
     
     
       20. The device of  claim 1 , wherein the maximum stress generated at the core of the glass plate by the thermal strengthening is at least 25 MPa. 
     
     
       21. The device of  claim 12 , wherein the glass comprises, by weight:
 55 to 65% of SiO 2 , 
 14 to 18% of Al 2 O 3 , 
 5 to 10% of B 2 O 3 , 
 5 to 17% of RO, and 
 at most 1% of R 2 O, 
 
       wherein RO is at least one alkaline-earth metal oxide selected from the group consisting of MgO, CaO, SrO and BaO, and R 2 O is an alkali metal oxide.